Flame detector using saturable core control

ABSTRACT

A system for detecting the presence of a burner flame in which the flame heats a temperature sensitive electrical device to produce an electrical current which is supplied to a winding on a rectangular hysteresis loop magnetic core to bias it to saturation in one direction termed the &#39;&#39;&#39;&#39;reset&#39;&#39;&#39;&#39; state, and in which means are provided to supply periodic current pulses to a winding on said core sufficient to saturate said core in the opposite direction termed the &#39;&#39;&#39;&#39;set&#39;&#39;&#39;&#39; state, and in which the core is provided with an output coil which generates a control pulse each time the core is shifted from the reset state into the set state. The presence of such control pulses indicates the presence of the flame and may be used to keep fuel flowing to the burner. A plurality of such cores are arranged to be individually controlled by a plurality of flames and means are provided to select the number and combination of the flames and cores to be controlled, including means for selectively connecting the outputs of the selected cores in parallel.

United States Patent Inventor William H. Hulsman Needham, Mass.

Appl. No. 46,155

Filed June 15, 1970 Patented Dec. 28, 1971 Assignee Columbia Gas System Service Corporation Columbus, Ohio FLAME DETECTOR USING SATURABLE CORE CONTROL Primary Examiner-Edward G. Favors Attorney-Russell & Nields ABSTRACT: A system for detecting the presence of a burner flame in which the flame heats a temperature sensitive electrical device to produce an electrical current which is supplied to a winding on a rectangular hysteresis loop magnetic core to bias it to saturation in one direction termed the reset" state, and in which means are provided to supply periodic current pulses to a winding on said core sufficient to saturate said core in the opposite direction termed the set state, and in which the core is provided with an output coil which generates a control pulse each time the core is shifted from the reset state into the set state. The presence of such control pulses indicates the presence of the flame and may be used to keep fuel flowing to the burner. A plurality of such cores are arranged to be individually controlled by a plurality of flames and means are provided to select the number and combination of the flames and cores to be controlled, including means for selectively connecting the outputs of the selected cores in parallel.

9 Claims, 4 Drawing Figs.

US. Cl 431/80 Int. Cl F23n 5/10 Field of Search 431/78, 80; 137/66 References Cited FOREIGN PATENTS 1,279,278 10/1968 Germany 431/80 23 FLAME MONITORING UNIT PATENTEI] M228 ml SHEET 1 0F 2 FIG. I

FLAME MONITORING UNIT IGNITER INVENTOR WILLIAM H.HULSMAN FIG. 2

ATTORNEYS PATENTEB B6228 I971 SHEET 2 OF 2 FIG. 3

FLAME MONITORING UNIT Slo 53 075 10 INVENTOR WILLIAM HJ'IULSMAN WNW FIG. 4

ATTORNEYS BACKGROUND OF THE INVENTION 1. Field of the Invention System for detecting the presence of one or more flames.

2. Description of the Prior Art The problem of detecting the presence or absence of a single flame or of a plurality of flames in a reliable and fail-safe manner has existed for many years. The prior art has produced a number of elaborate systems which suffer from the drawback of being large, expensive and in many cases of insufficient sensitivity to produce a positive response under all conditions which are likely to be encountered. Particularly where it is desired to select the number and combination of a plurality of flames, the complexity and cost of prior art systems have limited the types of devices to which they could be applied.

SUMMARY OF THE INVENTION In the present invention the limitations of the prior art have been overcome by suing one or more magnetic cores, each having a substantially rectangular hysteresis loop characteristic, as logic elements to indicate the presence or absence of a flame or flames and, if desired, to control the supply of fuel to one or more burners. Each core has a condition of saturation in one direction which is termed the set state and a condition of saturation in the opposite direction which is termed the reset state. Each core is supplied with a reset winding energized in response to a temperature sensitive element adapted to be heated by the presence of a flame in a burner, a reset winding adapted to be supplied with periodic pulses of current, and an output winding in which an output pulse is generated each time the core is shifted from one state to another. Such output pulses may be supplied through an amplifier to control the energization of a relay which controls the supply of fuel to said burner so that, as long asthe flame of said burner is present, the reset winding will be supplied with sufficient current to bias the core into its reset state, causing the set pulses to generate periodic output pulses which maintain the flow of fuel to the burner. Absence of the flame will cause the reset winding to be deenergized so that the core remains in one state and no output pulses are generated thus deenergizing the fuel control relay and cutting ofi the supply of fuel to the burner. I

In the case of a plurality of burners, a separate magnetic core of the above-described type is related to each burner.

The selection of which burners are to be monitored is determined by a separate fuel control valve, usually a manually operated valve, supplied with a switch to connect its associated output winding to a common control amplifier when the separate control valve is opened to admit fuel to its b..rner and which disconnects the output winding from such amplifier when the separate valve is closed to shut off the fuel supply to its associated burner. All of the output windings which are connected to the control amplifier are also connected in parallel with each other. As a result, the only cores which are operative to energize the control amplifier are the cores associated with those burners where individual valves are opened. The output of the control amplifier energizes a main valve which supplies fuel to all of the selected burners.

Appropriate igniter and delay devices, to carry out the required ignition of a single burner or of a plurality of burners and to maintain the system in operation, are provided.

' BRIEF DESCRIPTION OF THE DRAWINGS In the annexed drawings:

FIG. 1 is a simplified diagram of a single burner monitoring system incorporating this invention;

FIG. 2 is a detailed circuit diagram of the flame monitoring unit ofFIG. l;

FIG.'3 is a simplified diagram of a plural burner monitoring system incorporating this invention; and

FIG. 4 is a detailed circuit diagram of the flame monitoring unit of FIG 3.

DETAILED DESCRIPTION OF THE INVENTION In the embodiment of the invention shown in FIG. I, a burner 1, adapted to support a flame 2, is supplied with fuel, such as a combustible gas, from a supply line 3 through a solenoid valve 4 and a manual valve 5. The solenoid valve 4 is actuated by a solenoid 6 which, when supplied with energizing current at its terminals 7 and 8, opens the valve 4 and which, when the energizing stops, closes the valve 4. The manual valve 5 is opened and closed by any suitable manually operable 'control element such as a knob 9. When the knob 9 is operated to open valve 5, it actuates a switch 10. One terminal of switch 10 is connected by a lead 11 to ground, while the other terminal of said switch 10 is connected by lead 12 to one terminal of the energizing element of a relay 13, the other terminal of which is connected by lead 14 to one terminal 15 of a source of electrical current, the other terminal 16 of which is connected to ground. Switch 10 is of a well-known type which, when knob 11 is turned to open valve 5, closes for a short time of sufficient duration to cause relay 13 to become energized, after which switch 10 opens. Relay 13 is provided with an operator 17 connected to switch anns'l8 and 19. Switch arm 18 closes a circuit from terminal 15 to terminal 7 of solenoid 6, terminal 8 being connected to ground. Thus solenoid 6 is operated to open valve 4. Thereupon, gas is free to flow through valves 4 and 5 to burner I. At the same time, switch arm 19 closes a circuit from terminal 15 through an igniter 20 to ground, thus energizing said igniter. igniter 20 is of any well-known type which supplies igniting energy at its output terminals 21 to ignite the gas flowing from the burner l to start the flame 2. Relay 13 is a well-known type which, after it has been energized by the pulse of operating current due to the closure of switch 10, maintains its operator 17 in its closed position for a predetermined period of time and then releases said operator to its open position.

Located in a position to be heated by the flame 2 is a thermocouple 22, the output of which is fed to a flame monitoring unit 23, the details of which will be described below. As long as thermocouple 22 is heated by flame 2, unit 23 supplies energizing current to a solenoid 24 which in turn causes its operator 25 to move switch arm 26 to complete a path across the switch arm 18 and its cooperating contact. The time delay in the opening of relay 13 is of sufficient length so that by the time switch arm 18 leaves its contact, flame 2 will have started and will have heated thermocouple 22 to cause solenoid 24 to be energized, thus maintaining the circuit to solenoid 6 through the closure of switch arm 26 on its contact. Thereafter, should flame 2 be extinguished, thermocouple 22 will cool resulting in a deenergization of solenoid 24 which will open switch arm 26 to deenergized solenoid 6, thus causing valve 4 to close and shut off the supply of gas to the burner.

FIG. 2 shows the details of the flame monitoring unit 23. In FIG. 2, the terminals of the thermocouple 22 are connected to a winding 27 on a magnetic core 28. Magnetic core 28 is of the well-known type which has a substantially rectangular hysteresis loop characteristic whereby it can be magnetized to saturation in one direction which may be termed the set state and magnetized to saturation in the opposite direction which may be termed the reset" state. Such a core also possesses the property of magnetic retention which causes it to remain in either its set or reset state until it is subjected to a sufficient coercive magnetic force to shift it from one state to the other. Winding 27 is so related to core 28 that, when supplied with current from the output of a heated thermocouple 22, it biases the core into one of the above states which, for convenience, may be considered the reset state. A second winding 29 is also provided on the core 28 and is energized by periodic pulses of current which are of sufficient magnitude to insure that, during each such pulse, the magnetization of the core 28 will be put into a state opposite of that produced by the thermocouple 22, or into the set state.

The periodic current pulses for winding 29 are derived from a pair of terminals 30 and 31 adapted to be supplied from an alternating current source. A circuit extends from the terminal 30 through a rectifier 32, a relatively high resistance 33 (e.g. 39kQ), a condenser 34 (e.g. O.l pf.) winding 29 and a relatively low resistance 35 (e.g. 33(2) to ground lead 36. Terminal 31 is also grounded. A trigger device 37 is connected from the junction of resistance 33 and condenser 34 to the ground lead 36. Device 37 is of any well-known type such as a semiconductor device which remains nonconductive until the voltage across it rises to a predetermined breakdown value (e.g. 32 v.) at which point it becomes strongly conductive. It could be bilateral, as illustrated, or it could be in the form of a unilateral diode because the presence of diode 32 requires device 37 to break down only in one direction. In the above circuit, during each negative half cycle of the supply voltage, condenser 34 is charged through rectifier 32, resistance 33, winding 29 and resistance 35. The winding 29 is wound on core 28 in the same sense as winding 27 so that the direction of flow of the charging current for condenser 34 would tend to put the core into the reset state. However, due to the high resistance 33, the magnitude of such charging current is much lower than the current from thermocouple 22 and has virtually no effect on the magnetization of core 28 or the state of such magnetization. When, however, condenser 34 charges to the breakdown voltage of the trigger device 37, that device breaks down and completes a low-resistance discharge path for condenser 34, which thereupon discharges heavily, delivering a short high current surge through winding 29. Due to the sharpness of such discharge it imposes a relatively high voltage across said winding 29, causing a comparatively rapid change in the flux of the core. Also, the current surge is always high enough to insure that core 28 is saturated in the set state during such surge.

Magnetic core 28 is supplied with a third winding 38 in which an output voltage pulse is generated each time the flux in core 28 shifts rapidly from reset to set. The voltage generated by the thermocouple 22 resets core 28 during the decay of the surge in winding 29 so that it does not produce any appreciable output voltage pulse in winding 38, while the charging current for condenser 34 is so low and varies at such a low rate that it likewise does not produce any appreciable output in winding 38.

The result of the above arrangement is that when thermocouple 22 is cold, a single set pulse from the discharge of condenser 34 is enough to put the core 28 into its set state but, since there is nothing to cause it to shift out of that state, the core continues in its set state during all of the successive condenser 34 discharge pulses. Under these conditions no output pulses are generated in winding 38. When, however, thermocouple 22 is heated by flame 2, it generates sufficient current to bias the core 28 into its reset state between each ulse of discharge current from condenser 34. Therefore, during each successive pulse of discharge current from condenser 34, the flux of core 28 is rapidly shifted from the reset to the set state and a strong output pulse is generated in winding 38. Under this arrangement, the presence of a continuous series of periodic output voltage pulses in winding 38 indicates that flame 2 is present and the absence of such output pulses indicates that flame 2 has been extinguished.

It is to be understood that the term winding includes any arrangement of a conductor which will impart the desired magnetization to a core. For example, a typical arrangement may consist of a core 28 in the form of a small toroid of magnetic ferrite having three single pass-through conductors constituting the windings 27, 29 and 38.

The output from winding 38 preferably is used to control the solenoid valve 4. This is accomplished by the arrangement shown in FIG. 2 in which a condenser 39 is connected in series with a rectifier 40 and a lead 41 between terminal 30 and ground lead 36. Thus, condenser 39 is charged during each positive half cycle of the voltage at terminal 30 and 31. Switch actuator coil 24 is connected in series with a switching device such as a silicon control rectifier 42 directly across condenser 39. The control element 43 of silicon control rectifier 42 is supplied with firing pulses from an amplifier whose input consists of the output pulses of winding 38. That amplifier is of any well-known type and may consist of transistors 44 and 45, condensers 46, 47 and 48 and resistances 49 through 54 connected in the network arrangement as shown in FIG. 2. Since this is a well-known type of amplifier, its operation will not be described in detail herein. Its operation is such that a voltage pulse supplied from the winding 38 will cause a firing impulse to be supplied to the control element 43 thus causing silicon control rectifier 42 to fire and discharge condenser 39 through solenoid 24 to cause it to actuate its operator 25 to close switch arm 26 onto its adjacent contact. A rectifier is connected directly across solenoid 24 to cause a hold-in current to flow through solenoid 24 during the half cycle between successive discharges of condenser 39.

In summary, during each positive half cycle of the supply voltage, condenser 39 is charged and, as long as thermocouple 22 is heated by flame 2, the discharge of condenser 34, during each negative half cycle of the supply voltage, will produce an output pulse from winding 38 which, through the amplifier described, will cause silicon control rectifier 42 to fire, during each such negative cycle, discharging condenser 39 so as to operate the solenoid 24 and its associated solenoid valve 4 and keep fuel flowing to burner 1. When flame 2 is extinguished, thermocouple 22 cools and no current is supplied to winding 27 so that the output pulses from winding 38 cease. Therefore, silicon-controlled rectifier 42 is not fired, condenser 39 is not discharged, no energized pulses are supplied to solenoid 24 and solenoid valve closes so as to cut off the flow of fuel to burner 1.

In the embodiment shown in FIG. 3 and FIG. 4, where the elements are identical with those of FIG. 1 and FIG. 2, the same reference numbers are used with the addition of the subscript a." In FIG. 3, a plurality of burners are adapted to support flames 54, 55, 56 and 57 respectively, by being supplied with fuel from a common supply line 30 through the solenoid valve 44a. Each burner is individually provided with a manual valve 58, 59, 60 and 61 respectively. The manual valves are opened and closed by suitable manually operable control elements such as knobs 62, 63, 64 and 64 respectively. When knob 62 is operated to open valve 58, it actuates a switch 66, one terminal of which is connected by a lead 67 to ground, while the other terminal of said switch 66 is connected by lead 12a to the energizing element of a relay connected as described in connection with FIG. 1. Switch 66 is of the same type as switch 10 of FIG. 1. Thus, it will be seen that the arrangement of the switch 66 and its operations with respect to relay 13a is identical with that of switch 10 and relay 13 of FIG. 1.

The knobs 63, 64 and 65 are also provided with switches 68, 69 and 70 respectively, which are identical with switch 66 and are connected in parallel with switch 66. Therefore, the operation of any one or more of the knobs 62, 63, 64 or 65 to open their associate manual valves will initiate the operation of relay 13a, as described above for relay 13, which will cause solenoid valve 4a to open and to supply fuel to those burners whose manual valves 58, 59,60 and 61 have been opened. I

In FIG. 3 the switch arm 19a closes a circuit to an igniting device 71 which supplies igniting energy to pairs of igniting terminals 72, 73, 74 and 75 adjacent burners adapted to support flames 54, 55, 56 and 57 respectively. Therefore, those burners to which fuel is being supplied will be ignited to start their flames. Located in positions to be heated by flames 54, 55, 56 and 57 are associated thermocouples 76, 77, 78, and 79 which are connected by pairs of leads 80, 81, 82 and 83 respectively, through a cable 84 to input terminals of a flame monitoring unit 85, the details of which will be described below.

The knob 62, 63, 64 and 65 are also provided with switches 86, 87, 88 and 89 respectively. Each of these switches are of the type which close and remain closed as long as its associated knob has been turned into its valve opening position. Switches 86, 87, 88 and 89 are connected from ground to input terminals 90, 91, 92 and 93 respectively of flame monitoring unit 85.

The details of flame monitoring unit 85 are shown in FIG. 4. Flame-monitoring unit 85 is provided with a plurality of magnetic cores 94, 95, 96 and 97. Each identical with the magnetic core 28 as described in FIG. 2 and corresponding in number to the number of burners and flames to be monitored. The lead pairs 80, 81, 82 and 83 from thermocouples 76, 77, 78 and 79 are connected respectively to windings 98, 99, 100 and 101 each corresponding to the reset winding 27 of FIG. 2. Therefore, as long as a thermocouple 76, 77, 78 or 79 is heated by a flame, it will supply current to its associated winding 98, 99, 100 or 101 to bias its associated core 94, 95, 96 or 97 to the reset state. These cores are also provided with a second set of windings 102, 103, 104 and 105 respectively, each corresponding to the set winding 29 of FIG. 2. In FIG. 4, the windings 102, 103, 104 and 105 are all connected in series from the junction of the high resistance 33a and the trigger device 37a, through the condenser 34a, and a relatively low resistance 106 to ground lead 36a. Similar to the operation described for FIG. 2, the periodic dischargesof condenser 34a are of a magnitude to insure that, during each such discharge, the magnetization of all of the cores 94, 95, 96 and 97 will be put into the set state.

The cores 94, 95, 96 and 97 are also furnished with a third set of windings 107, 108, 109 and 110 each corresponding to the output winding 38 of FIG. 2. One end of each output winding is connected to a common conductor 111 leading through an appropriate resistor 112 to the emitter element of transistor 44a. The other terminals of windings 107, 108, 109 or 110 are connected respectively to the input terminals 90, 91, 92 and 93 previously described. For purposes of simplification of the diagram of FIG. 4, input terminals 90, 91, 92 and 93 are shown at two locations, but it is to be understood that, where the same reference numeral is used, the same terminal is involved. Thus, each manual valve which remains closed has its associated output winding disconnected from the system because the corresponding switch 86, 87, 88 or 89 is left open. However, for each manual valve which is opened, the corresponding switch 86, 87, 88 or 89 is closed and so the associated output winding 107, 108, 109 or 110 is connected to ground in parallel with all other output windings associated with opened manual valves.

Let us assume that knobs 62 and 64 are operated to open the corresponding manual valves 58 and 60. As previously described, closure of the corresponding switches 66 and 69 will cause relay 13a to be operated to open solenoid valve 4a and to cause ignitor 71 to be energized so that fuel will flow through the selected burners and flames 54 and 56 will be ignited. Thereupon, thermocouples 76 and 78 will be hea d to supply biasing current to windings 98 and 100 to bias cores 94 and 96 to their reset stages. Each discharge pulse from condenser 34a passes through in series through windings 102 and 104 associated with cores 94 and 96 and therefore, as long as thermocouples 76 and 78 remain heated, an output pulse will be produced in both windings 107 and 109 associated with said cores. It should be noted that, because of the series connection of windings 102 and 104, the shift of cores 94 and 96 from reset to set occurs at exactly the same time and therefore the voltage output pulses from windings 107 and 109 are ex actly in phase with each other. These output pulses combine to supply the input to the amplifier which causes the silicon control rectifier 42a to fire and operate solenoid 24a to keep solenoid valve 4a opened. In FIG. 4 the amplifier section may be provided with an additional resistor 112 and condenser 114 to adjust the operation of transistor 44a to the presence of the plurality of output windings 107 through 110.

If either flame 54 or flame 56 should become extinguished, its associated thermocouple will cool and the current generated by such thermocouple will fall below that required to bias its core into the reset stage. If, for example, thermocouple 76 cools, upon the occurrence of more than one discharge of condenser 340, there will be no shift of the flux of core 94 from the reset to the set state and no output voltage pulses will be generated in output winding 107. At the same time, output voltage pulses will tend to be generated in the output winding 109 because thermocouple 78 still is being heated by its flame 56. Due to the absence of simultaneous voltage pulses on winding 107, which is connected directly across winding 109, winding 107 constitutes a virtual short circuit across winding 109, thus preventing the voltage across winding 109 from building up to a value sufficient to actuate the amplifier to fire silicon controlled rectifier 42a. As a result, the solenoid 24a is deenergized and solenoid valve 4a closes to shut off the flow of fuel to all burners.

It should be noted that for those burners which are not selected by the operator to be supplied with fuel, the corresponding output windings are not connected across those output windings corresponding to the burners selected to be supplied with fuel. In the above example, manual valves 59 and 61 remain closed because knobs 63 and 65 remain in the closed position. Therefore, switches 87 and 89 are open and output windings 108 and 1 10 are disconnected from the other output windings 107 and 109. Thus, even though flames 55 and 57 are absent and thermocouples 77 and 79 are cool, the absence of output pulses in windings 108 and have no effect on the operation and, as long as flames 54 and 56 are present, the output pulses from windings 107 and 109 are permitted to reach the amplifier to cause solenoid valve 4a to remain open. The embodiment of FIG. 3 and FIG. 4, therefore, permits the monitoring of any number or combinations of the burners.

Various modifications of the embodiments described could be devised. In its broader aspects the invention does not require that the output pulses shall be generated on the shift of the core from its reset stage to its set stage, but such generations might be produced by a shift in the opposite sense. For example, the temperature sensitive device might operate to trigger periodic pulses of current, similar to those delivered by the discharge of condenser 34, to the reset winding, and the set winding might be supplied with biasing current similar to that supplied by thermocouple 22. However, the current to the set windings might be in the form of periodic pulses, which would be caused to occur in the opposite half cycles from those of the periodic pulses in the reset windings. Also, the temperature sensitive device does not necessarily have to be a thermocouple since other flame sensitive devices which can supply the requisite current to the reset winding may be used. Further, any reasonable number of burners may be monitored in accordance with the teachings of this invention. Additional variations will suggest themselves to those skilled in the art.

What is claimed is:

1. In a flame monitoring system having a burner adapted to support a flame a. a saturable magnetic core having a substantially rectangular hysteresis loop characteristic provided with three windings;

b. a temperature sensitive device adapted to be heated by the presence of said flame and to supply current to a first of said windings, when so heated, in a direction to saturate said core in a predetermined direction, constituting a first state;

c. means to supply periodic pulses of current to a second of said windings in a direction to saturate said core in the opposite direction to that of said predetermined direction constituting a second state;

d. the rate of change from one of said states to the other being of a magnitude to generate a substantial output voltage pulse in the third of said windings;

e. whereby said output voltage pulses are indicative of the presence of said flames.

2. The system as in claim 1 in which means are provided which is responsive to said output voltage pulses to control the flow of fuel to said burner.

3. The system as in claim 1 in which said temperature sensitive device is adapted, when heated, to apply a comparatively low voltage to said first winding and said means is adapted to apply a comparatively high voltage to said second winding;

' whereby each output voltage pulse is generated only upon a shift from said first state to said second state.

4. The system as in claim 3 in which said temperature sensitive device is a thermocouple.

5. The system as in claim 1 in which said means comprises: a. a condenser b. means for charging said condenser through a high-impedance circuit in series with said second winding and for periodically discharging said condenser through said second winding through a low-impedance circuit. The system as in claim 1 in which are provided: an alternating voltage source; a condenser;

means for causing said condenser during successive half cycles of said source of a given polarity;

d. means for causing said periodic pulses of current supplied to said second winding to occur during half cycles of said source of opposite polarity; whereby said output voltage pulses occur during said half cycles of opposite polarity;

. and means responsive to each of said output voltage pulses to discharge said condenser during a half cycle of said opposite polarity;

f. and means operative in response to the discharge of second condenser to control the flow of fuel to said burner.

7. In a flame monitoring system having a plurality of burners, each adapted to support a separate flame:

a. a separate saturable magnetic core associated with each of said burners;

b. each core having a substantially rectangular hysteresis loop characteristic provided with three windings;

. a separate temperature sensitive device associated with each burner and adapted to be heated by the presence of the flame of said burner and to supply current to a first of the windings of its corresponding core to saturate said core in a predetermined direction constituting a first state;

d. means to supply simultaneously periodic pulses of current to a second of said windings in a direction to saturate the associated core in the opposite direction to said predetermined direction constituting a second state;

e. the rate of change from one of said states to the other being of a magnitude to generate a substantial output voltage in the third of said windings of the associated core;

f. means for selecting the number and combination of said burners to be supplied with fuel;

g. means responsive to the operation of said last named means to connect to a fuel control device only those of said third windings which are associated with the selected burners;

h. and a common fuel supply for all of said burners under the control of said fuel control device;

i. whereby said system is responsive only to the presence or absence of flames at the selected burners.

8. A system as in claim 7 in which said last named means also connects the selected third windings in parallel with each other, whereby upon the occurences of an output voltage pulse in less than all of the selected third windings, the absence of a voltage pulse in the rest of the selected third windings prevents any of said output voltage pulses from actuating said fuel control device.

9. In a flame monitoring system having a burner adopted to support a flame;

a. a saturable magnetic core having a substantially rectangular hysteresis loop characteristic provided with three windings;

b. a temperature sensitive device adapted to be heated by the presence of said flame and to supply current to a first of said windings, when so heated, in a direction to saturate said core in a predetermined direction constituting a first state; c. means to supply current to a second of said windings m a direction to saturate said core in the opposite direction constituting a second state;

d. the current supplied to at least one of said windings being in the form of periodic pulses and the rate of change from one of states to the other being of a magnitude to generate a substantial output voltage pulse in the third of said windings;

. whereby said output voltage pulses are indicative of the presence of said flame.

k k i 

1. In a flame monitoring system having a burner adapted to support a flame a. a saturable magnetic core having a substantially rectangular hysteresis loop characteristic provided with three windings; b. a temperature sensitive device adapted to be heated by the presence of said flame and to supply current to a first of said windings, when so heated, in a direction to saturate said core in a predetermined direction, constituting a first state; c. means to supply periodic pulses of current to a second of said windings in a direction to saturate said core in the opposite direction to that of said predetermined direction constituting a second state; d. the rate of change from one of said states to the other being of a magnitude to generate a substantial output voltage pulse in the third of said windings; e. whereby said output voltage pulses are indicative of the presence of said flames.
 2. The system as in claim 1 in which means are provided which is responsive to said output voltage pulses to control the flow of fuel to said burner.
 3. The system as in claim 1 in which said temperature sensitive device is adapted, when heated, to apply a comparatively low voltage to said first winding and said means is adapted to apply a comparatively high voltage to said second winding; whereby each output voltage pulse is generated only upon a shift from said first state to said second state.
 4. The system as in claim 3 in which said temperature sensitive device is a thermocouple.
 5. The system as in claim 1 in which said means comprises: a. a condenser b. means for charging said condenser through a high-impedance circuit in series with said second winding and for periodically discharging said condenser through said second winding through a low-impedance circuit.
 6. The system as in claim 1 in which are provided: a. an alternating voltage source; b. a condenser; c. means for causing said condenser during successive half cycles of said source of a given polarity; d. means for causing said periodic pulses of current supplied to said second winding to occur during half cycles of said source of opposite polarity; whereby said output voltage pulses occur during said Half cycles of opposite polarity; e. and means responsive to each of said output voltage pulses to discharge said condenser during a half cycle of said opposite polarity; f. and means operative in response to the discharge of second condenser to control the flow of fuel to said burner.
 7. In a flame monitoring system having a plurality of burners, each adapted to support a separate flame: a. a separate saturable magnetic core associated with each of said burners; b. each core having a substantially rectangular hysteresis loop characteristic provided with three windings; c. a separate temperature sensitive device associated with each burner and adapted to be heated by the presence of the flame of said burner and to supply current to a first of the windings of its corresponding core to saturate said core in a predetermined direction constituting a first state; d. means to supply simultaneously periodic pulses of current to a second of said windings in a direction to saturate the associated core in the opposite direction to said predetermined direction constituting a second state; e. the rate of change from one of said states to the other being of a magnitude to generate a substantial output voltage in the third of said windings of the associated core; f. means for selecting the number and combination of said burners to be supplied with fuel; g. means responsive to the operation of said last named means to connect to a fuel control device only those of said third windings which are associated with the selected burners; h. and a common fuel supply for all of said burners under the control of said fuel control device; i. whereby said system is responsive only to the presence or absence of flames at the selected burners.
 8. A system as in claim 7 in which said last named means also connects the selected third windings in parallel with each other, whereby upon the occurences of an output voltage pulse in less than all of the selected third windings, the absence of a voltage pulse in the rest of the selected third windings prevents any of said output voltage pulses from actuating said fuel control device.
 9. In a flame monitoring system having a burner adopted to support a flame; a. a saturable magnetic core having a substantially rectangular hysteresis loop characteristic provided with three windings; b. a temperature sensitive device adapted to be heated by the presence of said flame and to supply current to a first of said windings, when so heated, in a direction to saturate said core in a predetermined direction constituting a first state; c. means to supply current to a second of said windings in a direction to saturate said core in the opposite direction constituting a second state; d. the current supplied to at least one of said windings being in the form of periodic pulses and the rate of change from one of states to the other being of a magnitude to generate a substantial output voltage pulse in the third of said windings; e. whereby said output voltage pulses are indicative of the presence of said flame. 